A known vehicle occupant protection system includes a vehicle occupant protection device, such as an air bag, and an inflator which provides inflation fluid to inflate the air bag. The air bag may be made of a gas permeable material and the inflation fluid may contain a quantity of nitrogen oxides (NO.sub.x) . It is desirable to reduce any NO.sub.x in the inflation fluid before the NO.sub.x can pass through the porous material of the air bag.